This invention relates to the fabrication of three-dimensional objects using extrusion-based layered manufacturing techniques. More particularly, the invention relates to forming three-dimensional objects from multiple types of modeling materials that are extruded in a flowable state and solidify after being deposited onto a base.
Three-dimensional models are used for functions including aesthetic judgments, proofing the mathematical CAD model, forming hard tooling, studying interference and space allocation, and testing functionality. Extrusion-based layered manufacturing machines build up three-dimensional models by extruding solidifiable modeling material from a nozzle tip carried by an extrusion head onto a base. xe2x80x9cWettingxe2x80x9d of the base by the extruded material serves to separate the modeling material from the tip. Movement of the extrusion head with respect to the base is performed in a predetermined pattern under computer control, in accordance with design data provided from a computer aided design (CAD) system. Examples of extrusion-based apparatus and methods for making three-dimensional objects are described in Valavaara U.S. Pat. No. 4,749,347, Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433, Crump et al. U.S. Pat. No. 5,503,785, Danforth, et al. U.S. Pat. No. 5,900,207, Batchelder, et al. U.S. Pat. No. 5,764,521, Swanson U.S. Pat. No. 6,004,124, Stuffle et al. U.S. Pat. No. 6,067,480 and Batchelder, et al. U.S. Pat. No. 6,085,957, all of which are assigned to Stratasys, Inc., the assignee of the present invention.
In the Stratasys FDM(copyright) three-dimensional modeling machines of the current art, the CAD design of an object is xe2x80x9cslicedxe2x80x9d into multiple horizontal layers by a software program. The machines then built up the object layer-by-layer by extruding modeling material in fluent strands, termed xe2x80x9croadsxe2x80x9d. Each extruded road has a thickness equal to the height of a slice. The material being extruded fuses to previously deposited material and solidifies upon a drop in temperature to form a three-dimensional object resembling the CAD model. The modeling material is typically a thermoplastic or wax material. Alternatively, other types of materials, such as metals, which become flowable when heated, which solidify upon a drop in temperature, and which adhere to the previous layer with an adequate bond upon solidification can be employed.
In a an extrusion-based modeling system, modeling material is supplied to the extrusion head as a feedstock of either a liquid or a solid material. Where the feedstock of modeling material is in solid form, a liquifier brings the feedstock to a flowable temperature for deposition. One technique is to supply modeling material in the form of a filament strand. Solid material feedstocks may alternatively be in the form of wafers, rods, slugs, or the like. A pressurization means is used to extrude molten modeling material from the extrusion head.
In modeling systems that employ a filament feed, modeling material is loaded into the machine as a flexible filament wound on a supply spool, such as disclosed in U.S. Pat. No. 5,121,329. The extrusion head, which includes the liquifier and a dispensing nozzle, receives the filament, melts the filament in the liquifier, and extrudes molten modeling material from the nozzle. Typically, the filament has a small diameter, such as on the order of 0.070 inches. A pair of motor-driven feed rollers on the extrusion head controllably advance the filament strand into the liquifier, which is heated so as to melt the filament. The liquifier is pressurized by the xe2x80x9cpumpingxe2x80x9d of the strand of filament into the liquifier by the feed rollers. The strand of filament itself acts as a piston, creating a xe2x80x9cliquifier pumpxe2x80x9d. The pressurization extrudes the molten modeling material out of an orifice of the nozzle at a volumetric flow rate, where it is deposited onto a base. The volumetric flow rate is a function of the size of the dispensing orifice and the rate of rotation of the feed rollers. By selective control of the feed-roller motor, the rate of advancement of the strand of filament, and thus the volumetric dispensing rate of the molten modeling material, can be closely controlled. A controller controls movement of the extrusion head in a horizontal x, y plane, controls movement of the base in a vertical z-direction, and controls the rate at which the feed rollers advance filament into the head. By controlling these processing variables in synchrony, the modeling material is deposited in roads at a desired flow rate, layer-by-layer, in areas defined from the CAD model. The dispensed material fuses and solidifies to form a three-dimensional object resembling the CAD model.
In building a model from a modeling material that thermally solidifies upon a drop in temperature, the modeling base is contained within a temperature-controlled build envelope. The build envelope is preferably a chamber which is heated to a temperature higher than the solidification temperature of the modeling material during deposition, and then gradually cooled to relieve stresses from the material. As disclosed in U.S. Pat. No. 5,866,058, this approach anneals stresses out of the model while is being built so that the finished model is stress free and has very little distortion.
In creating three-dimensional objects by depositing layers of solidifiable material, supporting layers or structures are built underneath overhanging portions or in cavities of objects under construction, which are not supported by the modeling material itself. For example, if the object is a model of the interior of a subterranean cave and the cave prototype is constructed from the floor towards the ceiling, then a stalactite will require a temporary support until the ceiling is completed. A support structure may be built utilizing the same deposition techniques and apparatus by which the modeling material is deposited. The apparatus, under appropriate software control, produces additional geometry acting as a support structure for the overhanging or free-space segments of the object being formed. Support material may be dispensed in a like fashion as the modeling material and in coordination with the dispensing of the modeling material, to build up supporting layers or a support structure for the object. Support material is deposited either from a separate dispensing head within the modeling apparatus, or by the same dispensing head that deposits modeling material. A support material is chosen that will adhere to the modeling material during construction, and that is removable from a completed object. Various combinations of modeling and support materials are known, such as are disclosed in U.S. Pat. No. 5,503,785.
To accommodate the dispensing of two different materials, the above-mentioned ""329 patent discloses a dispensing head having multiple supply passages into which materials of different compositions may be directed, with each passage terminating in a separate dispensing orifice. The dispensing orifices of the ""329 patent are arranged on a single broad-based nozzle tip, as shown in FIG. 11 thereof. Experimentation with broad-faced dispensing has tips taught, however, that a broad-faced tip drags against the road being extruded and smears out object features. Object corners are particularly problematic. If a corner does not get out from under a tip face before the tip changes directions, the tip face will drag against the corner. As the outer diameter of the tip face increases, the corner radius will increase as well, so that a narrow deposited road no longer makes fine features.
To overcome the feature smearing problems of multiple orifices, the above-mentioned ""785 patent teaches an extrusion head having independent nozzle tips. The apparatus of the ""785 patent uses an electromechanical method to move one tip higher or lower than the other, so that only the tip through which material is being extruded will contact the part surface. While the apparatus of the ""785 patent eliminates smearing problems, it introduces significant issues of alignment and calibration. Z-axis calibration of the tips must be verified, as well as X and Y offset calibration. Calibration requirements decrease throughput and reliability. The mechanical complexity of toggling the two tips additionally lowers reliability and increases cost of the machine.
Two other methods for accommodating the dispensing of two different materials are also known: (1) providing two extrusion heads, each including one dispenser for receiving and dispensing one of the modeling materials (such as is disclosed in the ""124 patent); and (2) providing a single extrusion head that dispenses two materials through a single orifice in a common dispensing tip, each material being provided to the tip from a separate flow path in the extrusion head (such as is shown in FIG. 6 of the ""329 patent). These methods have disadvantages as well. Where two extrusion heads are provided, cost and size of the modeling machine are increased. Additionally, tip calibration becomes more difficult. A single tip dispensing two different materials avoids calibration issues but introduces other problems. The tip needs to be purged of one material before the other material can be dispensed, in order to avoid mixing of dissimilar materials. Material as well as build time are wasted in purging the tip. Also, if any of a first material remains despite the purging effort, the remaining first material will contaminate the second material resulting in possible degradation of model quality.
None of the known methods for dispensing multiple materials from different material supply sources are entirely satisfactory. There is an unmet need for a dispensing apparatus that dispenses multiple types of modeling materials and is free from the disadvantages of the prior art.
The present invention is a multi-tip extrusion apparatus for three-dimensional modeling and a design methodology, whereby a single extrusion head carrying multiple dispensers having approximately co-planar, fixed-position tips is used to deposit multiple types of materials from distinct material supply sources. The tips are spaced apart a distance great enough so that a road of material extruded by a leading one of the tips will cool and shrink from the plane of the tips so that it is not smeared by contact with a trailing one of the tips. The apparatus of the present invention avoids the smearing, calibration, reliability, cost and throughput issues of the prior art techniques, enabling efficient production of good quality models. Additionally, by thermally insulating the dispensers from one another, the present invention may be utilized to dispense modeling materials having differing operating temperature ranges.